A light guide in the related art disclosed in Patent Document 1 will be explained with reference to FIG. 12. The light guide has a rectangular parallelepiped shape. FIG. 12 is a perspective view of a related-art light guide. The related-art light guide includes LED light sources 13 and a light guide 11 as shown in FIG. 12.
The light guide 11 includes a reflecting prism surface 22 having plural prisms 12 and a light emitting surface 23 opposite thereto. The light guide 11 allows incident light to propagate therein, allowing the light to be reflected by the prisms 12 and emitting the light from the optical light emitting surface 23. LED light sources 13 are arranged on both end portions of the light guide 11.
The LED light sources 13 are light generating means generating light emitted to the light guide 11. The depth of the prisms 12 becomes deeper as getting away from the LED light sources 13. The prisms 12 are formed in this manner so that light generated directly from the LED light sources 13 and reflected light reflected on the light emitting surface 23 can be uniformly reflected as the entire light guide 11 regardless of positions. That is, the light is difficult to reach inside as the light propagates inside, therefore, large prisms are used for reflecting light more efficiently.
FIG. 12 is schematically expressed and shows the small number of prisms 12. This is the same in the following drawings. In this case, for example, the reflecting prism surface 22 actually has 169 triangular prisms 12 in the light guide 11. These prisms 12 are numbered in a manner that N=1, 2, . . . , 84, 85, 84, 83, . . . , 2, 1 from an end portion of the light guide 11 to the center.
A length W of the light guide 11 is 59.2 mm and a thickness D of the light guide 11 is 3.5 mm. A pitch between the prisms 12 is fixed at 0.35 mm. As the depth of the prisms 12 (the height of triangles), a depth in the first prism 12 at the endmost position is 24.3 μm, and a depth of the 85th prism 12 in the center is 75 μm.
In the related-art light guide 11, the depth of the prisms 12 is changed to thereby allow the guided light to be reflected to the light emitting surface 23 side. Though the thickness D of the light guide 11 is 3.5 mm, a portion through which light is transmitted to the center is necessary, therefore, the maximum depth of the prisms 12 is 75 μm (a thickness of several %).
In the above structure, it is necessary to make the heights of the prisms 12 further deeper for designing a light guide 11 having a further larger area. However, it is difficult to reduce the thickness of the light guide 11 with respect to the heights of the prisms 12 to be less than a certain ratio, and the possible thickness is limited by the heights of the prisms 12. Accordingly, when a thin-film light guide 11 with 0.2 to 0.3 mm is to be designed, it is difficult to secure a thickness enough to guide necessary light.